Natural wetland systems have often been described as the "earth's kidneys" because
they filter pollutants from water that flows through on its way to receiving lakes,
streams and oceans. Because these systems can improve water quality, engineers
and scientists construct systems that replicate the functions of natural
wetlands. Constructed wetlands are treatment systems that use natural
processes involving wetland vegetation, soils, and their associated microbial
assemblages to improve water quality.
How do treatment wetlands
work?
Natural wetlands perform many functions
that are beneficial to both humans and
wildlife. One of their most important functions
is water filtration. As water flows through a
wetland, it slows down and many of the
suspended solids become trapped by vegetation
and settle out. Other pollutants are transformed
to less soluble forms taken up by plants or
become inactive. Wetland plants also foster the
necessary conditions for microorganisms to live
there. Through a series of complex processes,
these microrganisms also transform and remove
pollutants from the water.
Nutrients, such as nitrogen and phosphorous, are
deposited into wetlands from stormwater runoff,
from areas where fertilizers or manure have been
applied and from leaking septic fields. These
excess nutrients are often absorbed by wetland
soils and taken up by plants and microorganisms.
For example, wetland microbes can convert
organic nitrogen into useable, inorganic forms
(NO3 and NH,,) that are necessary for plant
growth and into gasses that escape to the
atmosphere.
Why build them?
Wetlands are some of the most biologically
diverse and productive natural ecosystems in the
world. While not all constructed wetlands
replicate natural ones, it makes sense to
construct wetlands that improve water quality
and support wildlife habitat. Constructed
wetlands can also be a cost-effective and
technically feasible approach to treating
wastewater. Wetlands are often less expensive to
build than traditional wastewater treatment
options, have low operating and maintenance
expenses and can handle fluctuating water levels.
Additionally, they are aesthetically pleasing and
can reduce or eliminate odors associated with
wastewater.
A Popular Idea
Designing and building
wetlands to treat
wastewater is not a new
concept. As many as
5,000 constructed
wetlands have been built
in Europe and about
1,000 are currently in
operation in the United
States. Constructed
treatment wetlands, in
some cases involving the
maintenance of
important wetland
habitat, have become
particularly popular in
the Southwest, where the
arid climate makes the
wetland habitat
supported by these
projects an especially
precious resource.
Wetland Plants
Wastewater
Gravel Substrate
Impermeable Liner
Wastewater
Water Level Control
Wetland plants and associated microorganisms treat wastewater as it flows
through a constructed wetland system.
How are they built?
Constructed wetlands are generally built on
uplands and outside floodplains or floodways in
order to avoid damage to natural wetlands and
other aquatic resources. Wetlands are frequently
constructed by excavating, backfilling, grading,
diking and installing water control structures to
establish desired hydraulic flow patterns. If the
site has highly permeable soils, an impervious,
compacted clay liner is usually installed and the
original soil placed over the liner. Wetland
vegetation is then planted or allowed to establish
naturally.
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Design and Planning
Considerations:
If planned and maintained properly, treatment
wetlands can provide wastewater treatment and
also promote water reuse, wildlife habitat, and
public use benefits. Potentially harmful
environmental impacts, such as the alteration of
natural hydrology, introduction of invasive
species and the disruption of natural plant and
animal communities can be avoided by
following proper planning, design, construction
and operating techniques. The following
guidelines can help ensure a successful project:
• Construct treatment wetlands, as a rule, on
uplands and outside floodplains in order to
avoid damage to natural wetlands and other
aquatic resources, unless pretreated effluent
can be used to restore degraded systems.
• Consider the role of treatment wetlands
within the watershed (e.g., potential water
quality impacts, surrounding land uses and
relation to local wildlife corridors).
• Closely examine site-specific factors, such
as soil suitability, hydrology, vegetation, anc
presence of endangered species or critical
habitat, when determining an appropriate
location for the project in order to avoid
unintended consequences, such as
bioaccumulation or destruction of critical
habitat.
• Use water control measures that will allow
easy response to changes in water quantity,
quality, depth and flow.
• Create and follow a long-term
management plan that includes regular
inspections, monitoring and maintenance.
This hog operation in Taylor County, Iowa, uses a wetland system constructed
on a series of hillside terraces to filter and purify wastewater. Water quality
tests indicated that the effluent from the treatment wetland was cleaner than
that required for wastewater treatment plants.
Tras t3iixs Projaol Irnprovos Writer QnnlHy
In 1990, city managers in Phoenix, Arizona, needed to improve the performance of
their 91st Avenue Wastewater Treatment Plant to meet new water quality standards
issued by the Arizona Department of Environmental Quality. After learning that
upgrading their treatment plant might cost as much as $635 million, the managers
started to look for a more cost-effective way to polish the treatment plant's wastewater
discharge into the Salt River. A preliminary study suggested that the city consider a
constructed wetland system that would polish effluent, while supporting high-quality
wetland habitat for migratory waterfowl and shorebirds, including endangered species,
and protecting downstream residents from flooding at a lower cost than retrofitting
their existing treatment plant. As a result, the 12-acre Tres Rios Demonstration Project
began in 1993 with assistance from the U.S. Army Corps of Engineers, the Bureau of
Reclamation and EPA's Environmental Technology Initiative and now receives about
two million gallons of effluent per day. The demonstration project was so successful
that the city and the Bureau of Reclamation asked EPA for help in expanding the
project to a full-scale, 800-acre project. For more information on the Tres Rios
Constructed Wetlands Project, visit, http://phoenix.gov/TRESRIOS/
EPA 843-F-03-013
Office of Water
August 2004
Treatment Wetlands (2004), Robert H. Kadlec and Robert L. Knight, Lewis Publishers, Boca Raton, Fl.
Guiding Principles for Constructed Treatment Wetlands: Providing for Water Quality and Wildlife Habitat (2000), United
States Environmental Protection Agency, EPA 843-B-00-003. Available online at www.epa.gov/owow/wetlands/
constructed/guide, html
Constructed Wetlands Handbooks (Volumes 1-5): A Guide to Creating Wetlands for Agricultural Wastewater, Domestic
Wastewater, Coal Mine Drainage and Stormwater in the Mid-Atlantic Region (1993-2000), United States Environmental
Protection Agency. Available online at www.epa.gov/owow/wetlands/pdf/hand.pdf
Handbook for Restoring Tidal Wetlands (2000), Joy B. Zedler, CRC Press, Boca Raton, FL.
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